1. Field of the Invention
The present invention relates to a fabrication process for a semiconductor optical device integrating a plurality of optical semiconductor elements, such as laser diode (LD) and so forth, on a common substrate.
2. Description of the Related Art
Application of semiconductor optical elements, such as a semiconductor laser, a semiconductor optical modulator, a semiconductor optical switch and so forth, to optical fiber communication, optical measurement, optical switching and so on has been widely studied. In the recent years, monolithically integrated semiconductor optical devices have been attracting attention as devices enabling down-sizing and lowering of cost of the devices and enabling significant improvement of optical coupling efficiency between elements.
Upon monolithic integration of semiconductor optical elements having mutually different applications and functions, it becomes necessary to form a plurality of regions having different bandgap energy in a common semiconductor substrate. This is due to difference in operation wavelengths (bandgap energy) of respective semiconductor optical elements. Typical conventional fabrication process of the monolithically integrated semiconductor optical device will be discussed hereinafter.
FIG. 1 is a section of an integrated semiconductor optical device integrating a distributed feedback laser (DFB) and an electroabsorption modulator reported by H. Soda et al. in "HIGH-POWER, AND HIGH-SPEED SEMI-INSULATING BH STRUCTURE MONOLITHIC EELECTROABSORPTION MODULATOR/DFB LASER LIGHT SOURCE", Electronics Letters, Vol. 26, Pp. 9-10 (1990) (first prior art).
In the fabrication process of the semiconductor optical device shown in FIG. 1, after providing a diffraction grating 201a in a laser side region on n-type InP substrate 201, an optical guide layer 203, an etching stopper layer 212 and a laser active layer 204a are grown as a laser region layer structure on the entire surface of the substrate.
Subsequently, laser active layer 204a in a region to form the modulator is removed by selective etching. Then, by burying regrowth, an optical absorption layer 204b forming a butt-joint structure with the laser active layer 204 is formed. Subsequently, on the entire surface, p-type InP clad layer 206 and a cap layer 207 are grown. Finally, by removing a part of the cap layer 207 and protecting the relevant portion with an insulation layer 211, p-side electrode 209 is formed on the epitaxial layer side, and n-side electrode 210 is formed on the substrate side.
While relatively high optical coupling efficiency higher than or equal to 80% can be attained between the laser and the modulator with the construction set forth above, sufficient controllability cannot be achieved in the etching and burying regrowth steps. Therefore, a problem is encountered in that satisfactory structures cannot be fabricated with reasonably high reproductivity.
As a solution for this, it is reported in M. Aoki et al, "NOVEL STRUCTURE MQW ELECTROABSORPTION MODULATOR/DFB-LASER INTEGRATED DEVICE FABRICATED BY SELECTIVE AREA MOCVD GROWTH"Electronic Letters, Vol. 27, pp. 2138-2140 (1991) (second prior art), a fabrication process of an integrated-type semiconductor optical device, in which optical waveguides for two regions can be formed by a single epitaxial growth process by utilizing a metal organic vapor phase epitaxy (MOVPE), thereby eliminating necessity of the burying regrowth process. Similar technology has been proposed in Japanese Unexamined Patent Publication (Kokai) No. Heisei 4-100291 (third prior art).
FIGS. 2A to 2C are perspective and sectional views showing sequence of steps in the fabrication processes as proposed in the second and third prior arts.
In the fabrication process proposed in both publications, as shown in FIG. 2A, a pair of SiO.sub.2 mask 302 (mask width is several tens to several hundreds .mu.m) serving as growth blocking mask are formed with a several tens .mu.m of interval (gap), are formed only on a laser region on a n-type InP substrate 301.
Subsequently, as shown in FIG. 2B, an optical guide layer 303, an active layer 304, a clad layer 306 and a cap layer 307 are sequentially grown by the MOVPE selective growth method on the portion not covered by the SiO.sub.2 mask 302 on the surface of the n-type InP substrate 301.
Next, as shown in FIG. 2C, a SiO.sub.2 mask 302a is selectively formed on the cap layer 307. With taking the SiO.sub.2 mask 302a as mask, 1.5 to 2.0 .mu.m width of optical waveguide is formed by performing mesa-etching for both of the laser region and the modulator region. Subsequently, with taking the SiO.sub.2 mask 302a as mask, Fe doped InP layer 313 serving as high resistance layer is formed at both sides of the optical waveguide.
Namely, in this prior art, an etching step of the semiconductor is required for forming the optical waveguide, precise control is required in the fabrication process to causing degradation of yield.
Kato et al., "DFB-LD/MODULATOR INTEGRATED LIGHT SOURCE BY BANDGAP ENERGY CONTROLLED SELECTIVE MOVPE", Electronics Letters, vol. 28, pp 153 to 154 (1992) (fourth prior art) reports other fabrication process of an integrated light source constituted of a DFB laser and an optical modulator.
In the fabrication process of the semiconductor device, growth blocking masks are formed both in the region I where the DFB laser is formed and in the region II where the modulator is formed, as shown in FIG. 3A. Namely, on the n-type InP substrate 401, a SiO.sub.2 mask 402 common to the regions I and II is formed. The mask width Wm is wider in region I than in region II. Here, a mask gap width W0 defining an active region is formed in equal width in both regions.
Thereafter, by MOVPE selective growth method, an optical guide layer, a multiple-quantum well (MQW) and p-type InP layer are formed.
In this prior art, utilizing the feature that the bandgap energy of the waveguide can be controlled by varying the mask width of the growth blocking mask upon MOVPE selective growth, a junction structure of the DFB laser and the optical modulator having approximately 100% of optical coupling efficiency can be realized by one growth sequence.
FIG. 3B is a graph showing a relationship between the mask width Wm and a bandgap wavelength of an active layer formed therebetween. As shown in FIG. 3B, by setting the mask width Wm of the growth blocking mask at 10 .mu.m on the laser side and 3 .mu.m on the modulator side, the bandgap energy can be varied as much as 58 meV (converted into the bandgap wavelength of 120 nm).
Also, in the fourth prior art, since the optical waveguide is directly formed by the MOVPE selective growth method, the step of forming the optical waveguide by mesa-etching of the semiconductor is unnecessary and thus the fabrication process of the integrated semiconductor optical device can be simplified. Therefore, it becomes possible to fabricate this optical device with high controllability and reproductivity.
In the first prior art (FIG. 1) reported by Soda et al., since a cut-and-paste like manner of selective etching and burying regrowth is employed, precise control of fabrication process is required to match the wavelength. Therefore, a problem of low reproductivity is encountered. Also, since two optical waveguides are formed by two separate epitaxial growth processes, increased number of process steps are required resulting in a low yield.
In the second and third prior arts (FIGS. 2A to 2C) as disclosed in Aoki et al. and Japanese Unexamined Patent Publication No. Heisei 4-100291, since two active layers are simultaneously formed through the single epitaxial growth step, reduction of number of process steps and improvement of yield can be achieved. In these prior arts, however, to realize an optical waveguide for the fundamental transverse mode, mesa-etching method is inherently used, leading to difficulty of reproductivity and causing a problem in uniformity of the products.
The process reported by T. Karo et al. (fourth prior art: FIGS. 3A and 3B), since the selectively grown active layer is used as is as the optical waveguide, the problems in the foregoing first to third embodiments can be basically solved. However, in the fourth prior art, since the bandgap energy difference between two the regions cannot be made large enough, application is limited.
The followings are the reasons why the bandgap energy difference between two regions cannot be made large. When mask width is varied, due to difference of decomposition rate of material species on the mask, crystal composition is varied to cause a problem of lattice-mismatch. For example, in case of the laser for the 1.55 .mu.m-band, the layer thickness of the MQW active layer is typically in the order of 0.1 .mu.m. In order to make the critical thickness at which crystal defects are introduced in the layer to be thicker than or equal to 0.1 .mu.m, lattice-mismatch should be restricted to be smaller than or equal to 15%. When the difference of the mask width is restricted to 10 .mu.m, for instance, effective bandgap energy difference possible to be realized, is derived by extrapolating the curve of FIG. 3B, to be 75 meV. Thus, in the fourth prior art, the bandgap energy difference between two regions cannot be greater than 75 meV.
As is known in the art, concerning the characteristics of the optical modulator, extinction characteristics is improved at greater optical confinement factor, that is at greater layer thickness. Whereas, in the fourth prior art, since lattice-mismatch is caused during selective growth, thick layers cannot be formed making it difficult to improve the extinction characteristics.
In the integrated semiconductor optical device, it is desired to form two regions having large bandgap energy difference on a common substrate. Greater bandgap energy difference is required for the following reason.
Currently, the wavelength of the semiconductor laser to be employed in optical communication is primarily 1.3 .mu.m band. In the future, it is expected that communication in the 1.55 .mu.m band, at which the loss in the optical fiber becomes minimum, will be increased. Accordingly, as a terminal for optical communication, it becomes necessary to cover both the 1.55 .mu.m band and the 1.3 .mu.m band.
On the other hand, in order to realize wavelength division multiplexing (WDM) communication, it is necessary to make it possible to input a plurality of semiconductor lasers oscillating at different wavelength into a single optical fiber. Then, in order to realize such a system by the monolithic semiconductor optical device, it is desired to combine plurality of laser lights into one using an optical waveguide, and subsequently, coupling them to an optical fiber. In this case, in order to suppress the optical propagation loss in the waveguide, it becomes necessary to set the bandgap of the waveguide to be sufficiently shorter than the respective laser oscillation wavelengths. Therefore, it is necessary to form a laser having a 1.55 .mu.m oscillation wave length, and a waveguide whose effective band gap wavelength is less than or equal to 1.3 .mu.m simultaneously. In terms of energy, the energy difference greater than or equal to 150 meV has to be realized.